Plotting devices of this type are primarily employed on drawing boards and are equipped with a drawing head on the traveling carriage which covers the drawing board, which accepts two rulers at right angles to each other in the drawing plane. Such plotting devices can also be used with horizontal drawing tables with coordinate collection instruments or similar instruments, and are intended to facilitate for the user the preparation of drawings or the collection of coordinates.
German Offenlegunschrift No. 2,717,399 discloses a plotting device of the initially mentioned type in which the coordinates of the actual position of the drawing head or of the intersecting point of the ruler edges are collected in a basic Cartesian coordinate system. The zero point on the drawing plane can be determined randomly. However, due to the construction of the plotting device, the axes of the coordinates are always fixed in the traveling direction of the two traveling carriages. The actual measured coordinate values (x, y) of the basic system are determined, with a desired scale factor included if desired, by indicator units which are located in the traveling carriage or the drawing head.
It is possible with this known device to establish the zero point of the basic coordinate system, hereinafter called the "basic" system, at any selected spot of the drawing plane and to determine the measured coordinate values, weighted with a selected scale factor. For the user, drawing aids are selected with respect to dimensioning which facilitate quicker preparation of the drawing. Aside from assistance in drawing parallel lines and in the dimensioning of the finished drawing, however, the user receives no further assistance. The draftsman is specifically tied to the basic coordinate system, whereby particularly those portions of the drawing which consist of circular parts or similar parts must be prepared without further drawings aids, which limits the applicability of these known devices.
More recently, computer-assisted drawing (CAD) methods have been employed in the preparation of manufacturing or production drawings. Within the scope of such CAD methods, computers and storage devices, such as disc storage units or punch tape devices, are employed in the preparation of production drawings, for the purpose of storing a visual representation of drawings in the form of numerical data. Printout terminals can be connected to the input connector units of the storage coordinate collection instruments and plotters. It is the objective of these CAD methods to convert, with the least possible amount of programming and storage expenditure, the designs or drawings which were manually prepared at the drawing board into a set of numerical data which are stored in known storage media and which, if needed, can be retrieved and transmitted in the form of control signals to numerically controlled machines, such as tooling equipment or plotters.
In order to realize the conversion of a drawing into numerical data with the acceptable programming and storage expense, a drawing is prepared at the drawing board and is, in general, completely dimensioned. Subsequently, the contours of the drawn object or of the represented individual parts, are entered into a memory bank in some form of descriptive language. As form-describing language, certain base line elements are used, such as circles, straight lines, etc., in the most appropriate coordinate system for describing the particular line element. In each instance, the type of line element and the coordinates of the final point of the line element, which is to be drawn from an actual point of the entire line, must be stored. The dimensions recorded on the production drawing must in most instances be entered separately. This working method is only possible, despite huge expenditure of time, if, depending upon the type of line element which must be described, the most appropriate system of coordinates can in each case be employed and if a free and unimpeded transition into various systems of coordinates is possible at any time.
Alternatively, designs or drawings prepared manually at the drawing board can be collected via coordinate collection instruments, and can be digitalized and stored in the form of numerical data. In this process, the lines are, for instance, collected through regularly spaced coordinate values. In this process also, the storage requirement is significantly reduced if the collection of data and their storage can be effected based in each case on a coordinate system adapted to the particular line element.
The drawback with these known processes lies in the fact that the drawing must first be made by hand and must be completely dimensioned, and subsequently, on another instrument, the lines which were drawn during the preparation of the drawing must essentially be redrawn or traced, or simulated via an input device, so that they can be stored. It is particularly disadvantageous that a change in the contour and dimensioning of a drawing and a corresponding change in the memory contents are cumbersome and time-consuming, since the drawing must constantly travel back and forth between drawing board and coordinate collection device or other input device.